FTB 07-21-2021: Well-Above Average Moisture Continues & Increases the Flood Threat

Issue Date: Wednesday, July 21st, 2021
Issue Time: 9:50AM MDT

MODERATE flood threat has been issued for the San Juan Mountains and portions of the Northern Mountains, Central Mountains and Front Range
LOW flood threat has been issued for the Northwest Slope, Grand Valley, Southwest Slope, San Luis Valley, Southeast Mountains and a portion of the Urban Corridor

The High continues to pull sub-tropical moisture northward on its west side, and its more southwest to northeast orientation today has also begun to push higher moisture to the east as well. PW values within this plume (shown in blue below) are greater than 1 inch, which is a good proxy for the flood potential from storms that develop. Specifically, PW at Grand Junction was measured at 1.15 inches, and better moistening within the surface layer from the last two days of rainfall was noted. This should allow rainfall rates to become more efficient today and raise the flood threat. Over Denver, PW has nearly doubled since yesterday to 1 inch, and it should rise a bit more throughout the day.

Subtle shortwaves were noted moving through the flow (orange “X’s”), which will help produce lift out in front of them and cause more widespread coverage of the rainfall producing storms. As far as steering flow, a slight uptick in steering flow was noted (north), but training storms are expected to increase the precipitation totals and still increase the local flood threat. Slightly slower steering flows over the central and southern mountains should allow for another round of heavy rainfall today. A Moderate flood threat has been issued for this area with a Low flood threat issued for most of western Colorado and the eastern high terrains. The westerly steering flow will likely push some of these storms into the southern Urban Corridor late this afternoon into this evening where they could drop a quick 1 inch of rainfall in 30 to 45min, so the Low flood threat has been extended into this area. Flood threats today include road flooding, local stream flooding and mud flows/debris slides over the steeper terrains. There are elevated flood threats also issued for the recent burn areas, so head over the Fire Burn Forecast (top of page).

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. If there is a threat, hover over the threat areas for more details, and click on burn areas to learn more about them. For Zone-Specific forecasts, scroll below the threat map.

Zone-Specific Forecasts:

Northern Mountains, Grand Valley, Northwest Slope, Southwest Slope, San Luis Valley & San Juan Mountains:

Another day of widespread rainfall is forecast for these regions with the higher accumulations again expected over the elevated terrains. There should be a little bit higher instability north today, which will allow some more vigorous storms to develop. Instability south is forecast to be similar to yesterday. With slower steering flows (south) and training storms (north), the flood threat returns.

Max 1-hour rain rates up to 1.10 inches will be possible and the majority of storms will likely dump between 0.60 and 1 inch of rainfall in 30 to 45 minutes. Therefore, a LOW flood threat has been issued. Further south, isolated storm totals up to 1.80 inches will be possible with max 1-hour rainfall rates up to 1.25 inches. A MODERATE flood threat has been issued for these regions. Flood threats today include road flooding, local stream flooding and mud flows/debris slides over the steeper terrains. Even the San Luis Valley could see some stronger storms this evening with max 1-hour rain rates up to 0.75 inches possible. Some light showers may linger near the western border through early tomorrow morning, but most storms should begin to dissipate just before midnight.

Primetime: 1PM to 2AM

Front Range, Central Mountains, Southeast Mountains, Palmer Ridge, Raton Ridge, Urban Corridor, Northeast Plains & Southeast Plains:

Since the moisture has moved east, thunderstorm and rainfall activity are expected to pick up in coverage and intensity this afternoon and evening. The southern Front Range and Central Mountains could see max 1-hour rainfall rates up to 1.25 inches with isolated totals up 1.50 inches possible. This could lead to local stream flooding as well as mud flows and debris slides. A MODERATE flood threat has been issued. A LOW flood threat has been issued for the northern Front Range and Southeast Mountains where max 1-hour rain rates may reach just over 1 inch. Some activity may spill into the adjacent southern Urban Corridor with westerly steering flows, so the LOW flood threat has been extended into the area where this would be most likely. Some outflow boundaries could initiate some additional storms over the Northeast Plains this evening (north) capable of moderate rainfall (up to 0.75 inches), but the Raton Ridge and Southeast Plains are expected to stay dry.

Primetime: 1PM to Midnight